ASK THE EXPERTS: PV Grounding

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I need to “Ask The Experts” to help solve the conflict I find between required PV array grounding, and grounding for lightning protection. We are building a grid-tied, 5 kW pole-mounted PV system with four trackers and a string inverter.

National Electrical Code (NEC) and inverter instructions call for the PV ground to be carried back to the inverter ground connection to provide ground-fault protection. That is a recipe for disaster with a lightning strike on the rack or modules, because it invites the lightning to seek its only way to ground at the AC mains panel ground—through the inverter, and through the house. The farm has been hit by lightning six times in the last 20 years, and to minimize damage, I’ve learned to liberally use surge protectors (MOVs and GTRs) with the right clamping voltage, and provide multiple paths to earth on ground lines, as close to the point of strike as possible.

For good lightning protection, it seems like the PV array ground wire should be bonded to a ground rod at each pole, tied to the array’s emergency disconnect switch box and that box bonded to its own ground rod. The ground wire should then be coiled into a choke and bonded to another ground rod just before it enters the house and goes to the inverter.

With our history of lightning strikes, a PV array and rack with multiple paths to ground—starting with the first ground as close to the panel as possible for maximum lightning protection—would seem to be more important than a single ground point at the inverter for GFDI. How do I deal with this dilemma?

David DeJong • Dickens, Iowa

I believe the confusion is over what “ground” is connected where. There are actually at least four different electrical terms that include the word “ground,” and it’s important to understand the distinctions between them.

The “equipment ground” is the typically green wire that travels through the conduit along with the current-carrying wires. It connects all of the metal parts of the equipment together, to make sure that one exposed metal part cannot be at a different potential than other metal parts. This is also what WEEB clips, module racking, and bare copper wire connecting module frames together, or bare wire inside of Romex, do. The equipment ground connects to the “grounding electrode” either directly, or indirectly through the “grounding electrode conductor (GEC).”

The grounding electrode is the point of connection to the physical earth—usually a ground rod, or a UFER (connection to rebar embedded in the concrete foundation). There can be more than one grounding electrode—in your case, you want one at each pole mount to give lightning the most direct path to earth.

The GEC connects the “system grounding point” to the grounding electrode. It can also connect between one grounding electrode and another, if you have multiple grounding electrodes. It may look similar to an equipment ground, but must follow NEC requirements for sizing, continuity, and how it is run through conduit. For example, the GEC must be continuous (or irreversibly spliced) between the system grounding point and the grounding electrode, even if it goes through other boxes. Equipment grounds are allowed to have junctions if they pass through a box.

In addition, the equipment ground can connect to the GEC and hence the grounding electrode at more than one point. If you have an outbuilding fed from a house, for example, the NEC requires that a second grounding electrode be installed at the outbuilding, with a grounding electrode conductor run between it and the grounding electrode on the house. Then, the equipment grounds in the outbuilding must be connected to the grounding electrode at the outbuilding. This is very similar to your case with multiple poles, where each one has its equipment ground connected to its individual grounding electrode.

The grounded conductor is a current-carrying conductor under normal operation—but the equipment grounds only carry current in case of a fault. For DC systems, the grounded conductor is usually the negative wire; in a SunPower system, it may be the positive wire. With some of the newer transformerless inverters, neither current-carrying conductor is grounded. For AC systems, it is the neutral wire. The grounded conductor should have only one connection to the GEC—at the system grounding point.

The system grounding point (aka “system bonding point”) is the place where the connection between the grounded conductor and the GEC is made. For a grid-tied PV system with multiple inverters, there will be one system grounding point for the AC system and one for the DC system of each inverter. In the case of a grid-tied inverter, the DC system grounding point is the internal ground-fault protection device (GFPD) fuse or GFPD system. This is what the NEC and the inverter manual are referring to when they say that there can only be one ground connection. To be more precise, it should say that only one DC system grounding point per inverter is allowed.

On the AC side of the system, there is also a system grounding point—usually inside of the main circuit breaker panel of the house, where a wire connects the grounding electrode conductor and the neutral, or grounded, conductor. This is why you’ll often see neutral and equipment ground wires sharing a busbar in a main AC panel—but it is not acceptable in a subpanel. You would have a second system grounding point on the same AC system if the equipment grounds and neutrals shared a busbar in the subpanel.

If you’re using an ungrounded inverter, your system will still have a grounding electrode, an equipment ground, a GEC, an AC system grounding point, and a grounded AC conductor. It is only missing the DC system grounding point and the grounded DC conductor. It still needs the rest for the internal ground-fault protection circuitry to do all of its tests to make sure there are not inadvertent connections between the high-voltage DC-carrying conductors and an exposed piece of metal that someone might touch.